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5.6.9. Magnetic Materials

Table of Contents

Magnetic response of matter

So far, magnetism has been described mainly through magnetic fields produced by currents and the forces they cause. Magnetic materials add a new idea, matter itself can respond to an external magnetic field. Some materials are only weakly affected, while others can become strongly magnetized and can even act like permanent magnets.

When a material is placed in a magnetic field, tiny magnetic effects inside the atoms may line up to some degree. This overall response is called magnetization. The strength and direction of this response depend on the material.

Microscopic origin

Electrons inside atoms are responsible for most magnetic behavior in materials. Two main sources matter. One is the motion of electrons around the nucleus, which behaves like a tiny current loop. The other is the intrinsic magnetic property called spin. Each atom can therefore act like a tiny magnetic dipole.

In many materials, these atomic dipoles point in random directions, so their effects cancel on average. In an external magnetic field, some alignment may occur. The amount and type of alignment determine the magnetic class of the material.

Atomic dipoles in a material

Main types of magnetic materials

The simplest classification for beginners includes diamagnetic, paramagnetic, and ferromagnetic materials.

Diamagnetic materials

Diamagnetic materials develop a very weak magnetization opposite to the applied magnetic field. They are therefore weakly repelled by a magnetic field. This effect exists in all matter, but in many materials it is hidden by stronger effects.

Examples include copper, bismuth, water, and graphite.

Paramagnetic materials

Paramagnetic materials have atomic dipoles that tend to align with the external field. Their magnetization points in the same direction as the field, so they are weakly attracted.

This attraction is usually small and disappears when the external field is removed.

Examples include aluminum, platinum, and oxygen gas.

Ferromagnetic materials

Ferromagnetic materials show a much stronger response. In these materials, neighboring atomic dipoles tend to align with one another, producing regions where many dipoles point in the same direction. These regions are called magnetic domains.

A ferromagnetic material can become strongly magnetized in an external field, and some magnetization may remain even after the field is removed. This is why permanent magnets are possible.

Common ferromagnetic materials include iron, cobalt, nickel, and some alloys.

Comparison of magnetic materials

TypeResponse to external fieldAttraction or repulsionMagnetization after field is removedExamples
DiamagneticOpposes fieldWeakly repelledNo lasting magnetizationCopper, bismuth, water
ParamagneticAligns with field weaklyWeakly attractedNo lasting magnetizationAluminum, platinum
FerromagneticAligns stronglyStrongly attractedCan remain magnetizedIron, cobalt, nickel

A material is called diamagnetic if its induced magnetization is opposite to the applied field.
A material is called paramagnetic if its magnetization is in the same direction as the applied field, but only weakly.
A material is called ferromagnetic if it can become strongly magnetized and may retain magnetization after the external field is removed.

Magnetic domains

A ferromagnetic sample is usually divided into many domains. Inside one domain, many atomic dipoles are aligned. However, different domains may point in different directions, so the whole sample may initially have little net magnetization.

When an external magnetic field is applied, domains aligned with the field grow, and others shrink. This gives a large magnetic response.

Magnetic domains before and after magnetization

Permanent magnets and temporary magnets

A temporary magnet becomes magnetized mainly while an external field is present. Many paramagnetic materials and soft ferromagnetic materials behave this way.

A permanent magnet keeps significant magnetization after the magnetizing field is removed. This happens in hard ferromagnetic materials, where the domain structure resists changing back.

This difference is important in applications. Soft magnetic materials are useful in transformer cores and electromagnets, where easy magnetization and demagnetization are needed. Hard magnetic materials are useful for fridge magnets, compass needles, and electric motors.

Magnetization and magnetic permeability

To describe how strongly a material responds, physicists use quantities related to magnetization and magnetic permeability. For many beginner problems, the key idea is simple, materials can make the magnetic field inside them different from the field that would exist in empty space.

A common relation is

$$
B = \mu H
$$

where $B$ is the magnetic field in the material, $H$ is the magnetizing field, and $\mu$ is the permeability of the material.

The permeability is often written as

$$
\mu = \mu_0 \mu_r
$$

where $\mu_0$ is the permeability of free space and $\mu_r$ is the relative permeability.

For diamagnetic materials, $\mu_r$ is slightly less than 1. For paramagnetic materials, $\mu_r$ is slightly greater than 1. For ferromagnetic materials, $\mu_r$ can be much larger than 1.

Important relation for magnetic materials:
$$
\mu = \mu_0 \mu_r
$$
If $\mu_r < 1$, the material is diamagnetic.
If $\mu_r > 1$ but only slightly, the material is paramagnetic.
If $\mu_r \gg 1$, the material is typically ferromagnetic.

Hysteresis

Ferromagnetic materials do not always follow the same path when being magnetized and demagnetized. Their magnetization depends on their history. This effect is called hysteresis.

If the magnetic field is increased and then reduced to zero, the magnetization may not return to zero. The remaining magnetization is called remanence. To reduce the magnetization back to zero, a reverse magnetic field may be needed. The required reverse field is called coercive field.

This behavior is essential for magnetic storage, permanent magnets, and many electrical devices.

Qualitative hysteresis loop

Temperature effects

Magnetic behavior depends on temperature. Thermal motion tends to disturb alignment of atomic dipoles. As temperature rises, ordered alignment becomes harder to maintain.

For ferromagnetic materials, there is a characteristic temperature above which ferromagnetism disappears. This is called the Curie temperature. Above this temperature, the material behaves more like a paramagnetic substance.

Higher temperature generally reduces magnetic alignment.
Above the Curie temperature, a ferromagnetic material loses its ferromagnetic order.

Everyday examples and applications

Magnetic materials appear in many devices. Iron cores in electromagnets strengthen magnetic fields. Transformer cores use soft magnetic materials because they magnetize easily. Permanent magnets are used in speakers, motors, compasses, and magnetic latches.

Credit cards, hard drives, and some sensors also rely on materials whose magnetic state can be changed and detected. In medicine and engineering, carefully chosen magnetic materials are used where strong or weak magnetic response is needed.

Key ideas to remember

Magnetic materials respond to external magnetic fields because atoms and electrons have tiny magnetic moments. Diamagnetic materials are weakly repelled, paramagnetic materials are weakly attracted, and ferromagnetic materials are strongly attracted and can remain magnetized. Ferromagnetism is explained by domain alignment, and its practical behavior includes hysteresis, remanence, coercivity, and temperature dependence.

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